This invention involves an improved valve gate in the form of an electrical resistive heater which acts as a valve to control the flow of plastic molding material in applications such as injection molding.
In the well-known injection molding process, thermoplastic material is melted to form a viscous liquid which is then injected under pressure into a mold cavity where it cools and solidifies. Solidification is accompanied by volumetric shrinkage, therefore necessitating the maintenance of a high packing pressure during cooling in order to achieve dimensional accuracy of the molded parts. As the plastic in the mold cavity cools, the packing pressure can decline, without any resulting change in dimensions of the molded plastic part.
It is customary to provide a gate or valve to shut off the plastic flow into the cavity once injection is completed and the packing pressure has been induced, in order to prevent plastic in the mold cavity, which is pressurized, from flowing back into the supply runner. Several techniques have been employed to accomplish this shut-off in the constricted area or gate immediately upstream of the mold cavity. One known technique is to cool the gate area so that the plastic in the gate area freezes once flow has essentially stopped due to filling of the cavity, thereby blocking further flow in either direction. This type of cooling system is continuous, providing an essentially constant temperature, so that no precise control of the plastic flow is possible.
Another general type of construction is the use of a movable pin placed in the flow path to close the gate. However, movable pins introduce substantial complexity accompanied by problems such as wear and misalignment of the pin and valve seat, thereby causing maintenance and reliability problems and expenses.
A thermal gate, which has been marketed by Spear System, Inc. of Chatsworth, California and described in U.S. Pat. No. 3,800,027 to Tsutsumi employs a stationary central axial pin in the constricted gate area. The pin has one heating element in its main body and a separately controlled heating element at its tip, the tip being located in the smallest portion of the flow passage. With the tip heater current on, to melt the gate open, plastic flows longitudinally along the length of the pin in the annular zone between the pin and the surrounding walls of the passage. When the tip heater current is turned off, the plastic freezes in the gate area. There are several disadvantages in the Tsutsumi construction. Some of the flowing plastic passing through the gate area contacts a cooled passage wall, while other portions of the plastic contact the heated wire. The resulting lack of uniform thermal history is often highly undesirable. Secondly, the cooling rate of the Tsutsumi system will be limited by the fact that the wire can only cool by conveying its heat through the plastic to the outer cooled passage walls, because the remainder of the core pin stays hot.
Another patent disclosing a construction somewhat similar to Tsutsumi is Yoshida, U.S. Pat. No. 4,516,927.
In conventional cooled gates, there is no precise control of the plastic temperature in the gate area. Inaccuracy in the control can result in premature freezing of the plastic in the gate before the mold is filled, commonly known as a "short shot". This problem can be remedied only by using higher pressures or higher temperatures. Higher pressures require the use of larger machines and result in higher residual stresses, whereas higher plastic and mold temperatures result in longer cycle times to cool the part, increasing production costs.
To avoid the waste of plastic in runner systems, which freeze along with the molded part and then have to be removed and recycled, it is common to continuously heat the supply manifold throughout the molding cycle. Plastic flow is constricted in the gate area, and only the very tip of the gate, immediately upstream of the cavity, is cooled. However, it is difficult to accurately confine the cooling area to the constricted gate, while maintaining the supply manifold adjacent thereto in heated condition. Undesired continued heating in the gate area may cause the molten material in the molded part close to the gate to remain heated longer than the balance of the molded part. Because crystalline and semi-crystalline plastic molding materials are very sensitive to their thermal history, this lack of precise temperature control in the gate area may result in undesirable physical properties of the resulting molded part.
Laminations of materials possessing differing thermal conductivity properties have been proposed for use in injection molding molds, to improve the physical properties of the molded part. Exemplary of such prior art are the patents to Yotsutsuji, et al., U.S. Pat. No. 4,225,109 (thin metal layer lining mold cavity, formed on layer of heat insulating material, to delay cooling of surface of molded part); Yang, U.S. Pat. No. 4,390,485 (thin layer of electrically resistive metal lining mold cavity to produce rapid heating thereof). Additionally, co-pending U.S. patent application Ser. No. 616,294 of Holden, Suh and Border discloses a variety of laminated constructions which are selected for their ability to heat rapidly by electric resistance heating and to cool rapidly upon termination of electrical current flow, with low thermal inertia and minimal thermal stresses. Such laminated constructions are suggested therein for use in controlling the thermal response of the surface of a mold cavity.
U.S. Pat. No. 4,717,522 to Border, et al. discloses a thermal gate for use in injection molding apparatus wherein a narrow flow passage is formed of a thin-walled tube of iron-nickel alloy which also functions as a resistance heater and which is surrounded by a thin sleeve of electrically non-conductive thermal insulating material having low thermal inertia. The resistance heater and surrounding insulating sleeve are formed of materials having very low and closely matched coefficients of thermal expansion. Flow of molding material is initiated by applying electrical current to the resistance heater, thereby melting the plastic within the tube, whereas flow is terminated by interrupting the electrical current, whereby the previously generated heat is quickly dissipated through the insulating sleeve to the surrounding mold body, thereby freezing the molding material within the passage. This type of construction, while capable of effective performance as a thermal gate, goes involve several disadvantages. The physical properties of the materials and the dimensions necessary to provide low thermal inertia make the thermal gate very fragile and subject to breakage during fabrication and use. Furthermore, the extremely thin wall of the resistive heater tube is subject to wear and ultimate rupture by the flow therethrough of molding compounds which contain abrasive ingredients such as glass fibers.
In the art of ceramic elements, silicon carbide radiant heaters or "glow bars" have been used, but to Applicants' knowledge, conduction type ceramic heaters have not been made from two separate mixed powders, where one functions as an electrtical conductor and the other as an electrical insulator.
Accordingly, it is a principal object of the present invention to provide an improved thermal valve gate which can be economically fabricated in a variety of shapes and sizes, which is reliable and durable in use, and which can be designed for use at a range of power levels.
It is a further object of the present invention to provide an improved electrical resistive heater device which is capable of rapid heating and cooling and which may be fabricated in a variety of shapes and sizes.